European patent EP-A-410500 (Agfa-Gevaert NV) discloses a diffusion transfer reversal process (hereinafter called "DTR process") for obtaining a lithographic printing plate in which an imaging element is image-wise exposed and subsequently developed using a developing liquid or activating liquid in the presence of a silver halide complexing agent. The principles of the DTR process are known and have been described, for example, in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York (1972). In the DTR-process, non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called "silver solvent" into soluble silver complex compounds which are allowed to diffuse into an image-receiving layer of an imaging element and are reduced therein with a developing agent generally in the presence of physical development nuclei, to form a silver image having reversed image density values with respect to the silver image obtained in the exposed photographic material.
In the processing of sheet material, such a sheet material is brought into contact with aqueous processing liquids, and rinsing water in a processing machine. In a known processing machine there is an open-topped tank divided into a number of compartments, in each of which is located at least one pair of guide rollers to define a transport path for the sheet material. Processing liquid circulates through each compartment.
The reaction between the sheet material and the processing liquid takes place in a thin reaction zone or layer of the liquid close to the surface of the sheet material. As the reaction proceeds, active chemicals in the reaction zone become used up and this zone becomes saturated with reaction products. It is necessary for fresh active chemicals to diffuse into this zone from the bulk of the liquid and for the reaction products to diffuse away. To achieve this continuous refreshment of the reaction zone, circulation of the processing liquid is often employed.
There is a need for a variety of reasons to employ processing apparatus which is as small as possible and which uses as small a volume of processing liquid as possible. Nowadays, volumes as low as 0.25 l/m.sup.2 are now desirable, generating a need for even more effective refreshment of the reaction zone. While this may be achieved by increasing circulation rates, it is essential that the refreshment of the reaction zone is as uniform as possible over the width of the sheet, otherwise inconsistent processing results.
In order to obtain an aluminium based lithographic printing plate according to the DTR process generally at least three different liquids are used i.e. a developing or activating liquid, a rinsing liquid and a finisher liquid.
The developing liquid comprises a developing agent or a mixture of developing agents present in an alkaline processing solution and or these agents may be present in the photographic silver halide emulsion layer material, in which case the processing solution can be merely an aqueous alkaline solution that initiates and activates the development.
The imaging element is then guided through a diffusion transfer zone so that the silver halide complexes formed during the development step are allowed to diffuse into the image receiving layer where they are converted to silver. When the imaging element leaves the diffusion transfer zone a silver image is formed in the image receiving layer. The now useless photosensitive layer and optional other layers above the image receiving layer are then removed by guiding the imaging element through a washing and through a rinsing station.
In order to improve the printing properties, it is common practice to subject the imaged surface of the aluminium support to a chemical treatment that increases the hydrophilicity of the non-silver image parts and the oleophilicity of the silver image. This chemical after-treatment is preferably carried out with a lithographic composition called a "finisher", which comprises a compound enhancing the ink-receptivity and/or lacquer receptivity of the silver image, and also comprises at least one compound that improves the ink repelling characteristics of the hydrophilic supports. As disclosed in European patent application EPA 94200941.6 (Agfa-Gevaert NV) filed 8 Apr. 1994, such a finisher may comprise hydrophobing agents, surface-active agents and polyglycols, especially polyglycols in which ethylene oxide and/or propylene oxide groups predominate.
Not only is it necessary that each processing liquid used in the process is applied evenly over the surface of the printing plate, it is also necessary that the processing solutions be applied in a consistent manner, i.e. uniformly with respect to time. Some processing solutions, particularly the finisher liquid, may contain less soluble substances which tend to become deposited at various parts of the apparatus if the apparatus is not used for a period of time. For example liquid flow apertures may become blocked in this way, resulting in unpredictable liquid flows when the apparatus is next used.
It is an object of the present invention to provide means whereby the supply of processing liquid may be achieved in a substantially even manner over the surface of the sheet material being processed.
It is a preferred object of this invention to provide means whereby the supply of processing liquid is achieved in a substantially uniform manner, reducing the risks of the liquid supply means becoming blocked by the deposit of less soluble substances.